It is known from the practice that monitoring of the groove depth in the tyre tread is important for the safety of driving. Nevertheless, the driver needs not know the exact value of the groove depth, it is sufficient, if he/she is acquainted with the tyre tread wear in certain stages, which for a private car tyres are preferably in the range from 2 mm to 8 mm in steps of 2 mm, i.e. the groove depth should be indicated in steps of 8 mm, 6 mm, 4 mm. It is important that the information should be easily readable, it should be unambiguously expressed, and it should catch eye at a glance on the tyre tread.
From GB 2376002, there is known a tyre having an indicator embedded in the tread part under the surface. The indicator consists of rubber and is brightly coloured to contrast with the tread, to enable visual warning appearing at the surface when the tread is worn off. The indicator is located on the whole tyre circumference and may take the form of a band or several bands or pins passing through the tread depth. The indicator should be located at the minimum allowed depth of the tyre tread pattern or slightly over this allowed tread pattern depth.
WO2004/045872 utilizes the same principle of tyre wear indicator as described in GB 2376002.
U.S. Pat. No. 6,883,567 provides a method of measuring tyre tread wear by using a tyre, in which a marked part of the tread surface is created. The marked part changes its surface shape in dependence on the wear progress. The image data are scanned and processed and based on reference data entered in advance and the corresponding measured data on the tread surface wear, suitability for further using the tyre is determined.
EP 0250113 describes detection of early non-uniform tyre tread wear which is caused by wheel imbalance. The tyre tread is provided with groups of sipes of various depths so that with continuing wear also the number of sipes varies. The group of sipes preferably consists of at least five sipes on the outer ribs adjacent to the shoulder part of the tyre at the same circumferential position. The sipes are preferably formed with various lengths corresponding to their depth for the purpose of simpler identification. By observing how quickly the sipes of one group on the shoulder rib disappear by wear compared with the sipes on the second shoulder rib, non-uniform wear can be detected in early stage.
EP 1 066 991 describes a tyre, the wear of which can be observed by means of indicators arranged upwardly one after another, where each indicator is formed by a separate sipe running into the tyre in the tread direction. The sipe cross-section represents a number, which is at least 5.5 mm high and 3.5 mm wide.
The main disadvantage of this solution is the fact that individual indicator sipes are small, poorly visible from a greater distance, and they must be separated one from the other by sufficiently large spaces, which are at least as large as is the indicator height because of the necessity of good introduction of rubber in the mould at pressing, i.e. at displaying the groove depth steps of 8, 6, 4, the overall height of the indicator is at least 27.5 mm, which makes impossible to use the indicator in tread blocks which are narrower than 30 mm. Moreover, at the places where the indicator is used it is necessary to omit several transverse capillary sipes which are used mainly in winter tyres to improve their grip properties.
The tread groove depth is indicated by successive disappearing of indicators from the greatest number to the smallest one, while the remaining steps are always displayed. If one is not sufficiently informed about the indication method, this may possibly lead to inaccurate interpretation of the remaining groove depth. The sipe thickness is not sufficient, and when viewing it from a greater distance and with a soiled tread the indicator may be poorly visible. Because of complicated shape of the indicators, from the point of view of the production it is not possible to use thicker metal sheet of the forming lamella than 0.4 mm.
A disadvantage of the above solution is that each indicator number is formed by a separate lamella, each of them having a shape which is complicated to produce. Also disadvantageous is the fact that the groove depth is expressed by the greatest visible number, while the remaining depths are also visible, thus causing ambiguous depth expression, the numbers are small (height of 5.5 mm, width of 3.5 mm) and, thus, they are visible only at detailed tyre inspection. The indicator, which expresses the depths 8, 6, 4, is at least 27.5 mm high, the width is 3.5 mm, the thickness is less than 0.5 mm. A disadvantage is also the overall indicator height of at least 27.5 mm (structurally it cannot be lower), and therefore, the indicator cannot be used in treads which do not contain a circumferential band in the middle of the tyre tread or where the bands and blocks are narrower than 30 mm in the tangential direction.
The aim of the present invention is to produce a tyre, which is provided with a safety indicator indicating the groove depth, based on which one can simply estimate the remaining groove depth and thus the wear state of the tyre.
The basic aim has been to achieve an indicator which is better visible than described above, of which one can comfortably deduct the wear value at a routine tread check and where a warning possibility exists also at an incidental glance at the tread.